/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineMessageInterface.h
Send a message from the currently logged in user to a list of recipients
true if request was started
bool SendMessage ( int32 LocalUserNum, const [TArray](API\Runtime\Core\Containers\TArray)< FUniqueNetIdRef > & RecipientIds, const [FString](API\Runtime\Core\Containers\FString) & MessageType, const [FOnlineMessagePayload](API\Plugins\OnlineSubsystem\Interfaces\FOnlineMessagePayload) & Payload )